Effects of oxygen tension on the microbial community and functional gene expression of aerobic methane oxidation coupled to denitrification systems.

Aerobic CH oxidation coupled to denitrification (AME-D) can not only mitigate the emission of greenhouse gas (e.g., CH) to the atmosphere, but also reduce NO and/or NO and alleviate nitrogen pollution. The effects of O tension on the community and functional gene expression of methanotrophs and denitrifiers were investigated in this study. Although higher CH oxidation occurred in the AME-D system with an initial O concentration of 21% (i.e., the O-sufficient condition), more NO-N was removed at the initial O concentration of 10% (i.e., the O-limited environment). Type I methanotrophs, including Methylocaldum, Methylobacter, Methylococcus, Methylomonas, and Methylomicrobium, and type II methanotrophs, including Methylocystis and Methylosinus, dominated in the AME-D systems. Compared with type II methanotrophs, type I methanotrophs were more abundant in the AME-D systems. Proteobacteria and Actinobacteria were the main denitrifiers in the AME-D systems, and their compositions varied with the O tension. Quantitative PCR of the pmoA, nirS, and 16S rRNA genes showed that methanotrophs and denitrifiers were the main microorganisms in the AME-D systems, accounting for 46.4% and 24.1% in the O-limited environment, respectively. However, the relative transcripts of the functional genes including pmoA, mmoX, nirK, nirS, and norZ were all less than 1%, especially the functional genes involved in denitrification under the O-sufficient condition, likely due to the majority of the denitrifiers being dormant or even nonviable. These findings indicated that an optimal O concentration should be used to optimize the activity and functional gene expression of aerobic methanotrophs and denitrifiers in AME-D systems.